A photolithography apparatus is essentially comprised of a light source (the existing mainstream photolithography apparatus uses ArF-excimer lasers producing ultraviolet (DUV) light with a wavelength of 193 nm), optical illumination systems, projection lenses, a mask stage for holding a mask, a wafer stage for holding a silicon wafer, and vibration damping apparatuses. It requires many important parts, such as metering systems, exposure systems, zero-position sensors, alignment sensors, light intensity sensors and energy sensors, to have minimal vibrations, in order to create a “quiet” environment for some crucial modules. This is because vibrations can pass to a measurement frame to cause undesirable movements thereof, which will in turn disturb metering systems of the wafer and mask stages and thus increase errors in wafer and mask stages and surface errors. As a result, additional errors will be introduced in overlay and critical dimension (CD). Therefore, as one of key devices guaranteeing the performance of integrated circuit fabrication, vibration dampers are typically used to create an internal environment for the important parts inside the photolithography system, which is independently isolated from the external remainder thereof including the basic frame.
Early photolithography apparatuses used rubber dampers. With the theory of using air springs for vibration damping substantiated in the early 1980's, air spring-based passive damping techniques have been used in photolithography apparatuses for this purpose. Currently, most mainstream photolithography apparatuses employ air spring-based active damping techniques and an active controller-based active control strategy. In order for high-precision positioning compensation of a vibration isolation platform, these apparatus mostly use velocity sensors (geophones) for velocity measurement for damping feedback and compensation, position sensors for real-time position measurement of the vibration isolation platform, and voice coil motors for compensation for high bandwidth response. In addition, a gravity compensation means for supporting the vibration isolation platform employs a pneumatic control valve to compensate the pressure of an air bag inflated with compressed air in a real time manner for enabling the vibration dampers to withstand large loads and to effectively isolate and damp vibrations.
Typically, vibration dampers are arranged in a photolithography apparatus in three groups, each equipped with a vertical compensation motor and a horizontal compensation motor, along with a vertical measurement sensor and a horizontal measurement sensor, for vibration damping and isolation in six degrees of freedom, as well as the desirable positioning in the suspended state, of the vibration isolation platform.
Research efforts on vibration dampers aim at achieving a high load-bearing capacity, a low stiffness, a low resonance frequency and a high damping rate. These parameters can result in a low vibration transfer and minimize the influence of external disturbance to the internal environment. However, with the resolution of photolithography systems being continuously improved, more and more strict requirements are being imposed on the critical dimension achievable by the systems. In addition, with the continuous increasing of their throughput as well as the speed and acceleration of their wafer and mask stages, internal modules of photolithography apparatuses have been increasingly complicated and their overall weight has increased to 2-14 tons, even to 40 tons for TFT systems. Therefore, photolithography apparatuses are subjected to increasingly stricter requirements concerning the vibration damping performance of their “quiet” environment, and it is necessary for vibration damping devices to achieve high load-bearing capacities. Further, a concept of “negative stiffness” has been proposed in this art, and some foreign researchers have proposed a number of new vibration damping schemes based on magnetically levitated bearings.
In 2003, IDE (Integrated Dynamics Engineering) Inc. (Germany) filed a patent application (U.S. Pat. No. 7,290,642) for a magnetic spring device with negative stiffness. In this application, it was proposed for the first time to construct a vibration damping device using three permanent magnet poles, any adjacent two of which form a magnetic attractive force, thereby suspending a vibration-damped platform with bi-directional stiffness, as shown in FIG. 1.
In 2009, Professor Lomonova, Eindhoven University of Technology (TU/e), the Netherlands, proposed a passive damping device achieving suspension based on permanent magnet arrays. The device has a structure composed of two stacked magnetic arrays, and each array has a topological structure formed of a planar N-S magnet array. In this device, a vibration-damped platform is suspended based on an upward pulling force exerted by magnet arrays in the upper two layers because of magnetic attractive forces and on an upward thrusting force exerted by magnet arrays in the lower two layers because of magnetic repulsive forces. This device is capable of compensating for a maximum load of thousands of kilograms, and has a spring for vertically supporting the load with a stiffness of dozens of Newton per millimeter, which further assures a low resonance frequency.
While such passive magnetic suspension vibration damping devices are undoubtedly a significant technical advance over the conventional air floatation vibration damping devices, they suffer from inefficient utilization of magnetic energy for magnetic suspension at the same magnetic energy product generated by permanent magnets and relatively significant magnetic leakage, which are significantly detrimental to the application of photolithography apparatuses using them.